EMI shielding - some questions

EMI shielding

Hi,

I'd like to shield my PCB from EMI using a conductive painting in the plastic case. Does it work ? I should connect the shielding and my ground ? Directly ? Using a resistor ? a capacitor ?

I think that it's a really obscure theme.

Thanks.

Re: EMI shielding

Probably the result would be not very good. This conductive paint, probably will atenuate some of (very)high frequency components, but in general will not introduce considerable effect.

Re: EMI shielding

Your conductive paint will provide some shielding, and it should be directly connected to your circuit ground as close to the power supply ground return as possible. You want the connection point close to the power supply to help minimize the chance of shield noise modulating a signal return.

Whether or not it will provide enough shielding depends on many factors.

* What frequencies are you trying to shield against? - Your paint shield will work better against higher frequencies than it will against lower frequencies; however, higher frequency interferrence can find its way in through connector penetrations, power supply cables, etc.

* Is the potential source of EMI primarily RF or is it magnetic? - A simple conductive paint shield may stop high freqency RF, but it isn't going to keep out the magnetic field from an adjacent motor or power transformer.

* What power are the signals that need to be shielded against? - Your single conductive paint coating isn't going to keep out the 50Kw radio station on the next block. Large EM signals mean large currents flowing in the shield and the chance of secondary EM effects on the shielded circuit.

* How far from the shield will your circuit be? - EM fields follow the inverse square law for attenuation. Whatever your shield doesn't completely absorb may be weakened enough that shield-to-circuit spacing eliminates what's left. Another consideration is the capacitance between the shield and your circuit - if it's too close, you may detune or cross couple circuits on your own circuit board.

The answer could go on-and-on because the subject is a complex one. There are hundreds of books, and graduate level courses, aimed at EMC and EMI. The bottom line is that your painted shield won't hurt, and will probably help you somewhat - use it.

Re: EMI shielding

Wow, I really liked your complete and yet simple answer.

So, if I understood using a metal shielding can be far better than just the painting ? But how can I avoid interference to find it's way thru holes in the shielding (like a display ) ?

My equipment must be abe to still working while being hit by pulses like the ones defined in the IEC61000-4-2,4,5.

Can you suggest some books or better ebooks about that matter, if possible that don't requires a phd in physics to be usefull.

Re: EMI shielding

There is a nice book on that subject

Re: EMI shielding

I'd like to shield my PCB from EMI using a conductive painting in the plastic case

Click to expand...

Hi mscaff,

Please note, shielding is the LAST resort and most be done ONLY if those measure dont have expected efect:

-fine EMI measures in PCB drawing (ground planes, low impedance path to ground, ferrite beads etc)

-try to put some extra decoupling capacitors in your signal path (10pF-1nF) to improve EMI performance, without altering functionality

If U aply those measures corectly, maybe the shielding is even not necesary!!

Good loock!!

Re: EMI shielding

If the conductive painting is not uniform on the case, it would not work well.

EMI shielding

I agree with what mandrei said.
If you note and obey the EMC rules during Pcb design, you will have a better performance.
If you can't modify PCb,accordin my exp,shielding need to use a single shielding ground,may not connect to the signal ground or power ground.

Re: EMI shielding

It will definitely depend on frequencies of EMI, if it really is a problem. The conductive paint you refer to (if its the correct one) will also have nickel powder particles in the suspension and will provide magnetic shielding aswell, however in mass production it is really a last resort. The manufacturing implications of using these toxic sprays/paints to shield from EMI (when better PCB layout or beads etc. could also solve any EMI problems) are usually quite adverse and not the best path unless it is really needed. For one off unit it might just do quite well. Good luck !